Devices for inking lithographic printing plates generally comprise a plurality of form rolls which contact a printing plate. Each of the form rolls is usually in rolling contact with one or more vibrator rollers to which ink is applied by a large number of rollers, generally twenty or more, arranged in pyramid fashion.
Inking systems currently in use generally have rollers in the ink train of varying diameters, some of which vibrate longitudinally in an effort to eliminate ghosting and to provide desired quantities of ink to the printing plate.
The quantity of ink supplied through a train of rollers to a printing plate is generally controlled by adjusting ink keys and controlling dwell time of ductor rollers to control the input of ink to the long train of rollers. Heretofore, on a press for printing sheets thirty-eight inches wide about sixty individual inker adjustments had to be correlated. Changing a first adjustment required a change of a second which in turn required a third and usually readjustment of the first. The effect of such a change was not apparent on printed sheets for about five minutes and thus resulted in wasting excessive quantities of paper, sometimes five hundred sheets or more, while the operator adjusted the ink train by trial and error.
Since ink is applied by form rollers only to image areas of the plate, form rollers have a memory because ink accumulates on areas of the form rollers which contact non-image areas of the plate. The operator is faced with the impossible task of adjusting the inker to feed ink to the areas of the form rolls corresponding to image areas while attempting to minimize accumulation on areas corresponding to non-image areas. As a result, part of the image areas are starved and undesirable accumulation results on other parts of the rollers.
U.S. Pat. No. 3,283,712 describes an inking system devised to overcome ghosting. The system comprises two rollers urged together in pressure indented relation, opposite surfaces of adjacent rollers moving in opposite directions. All of the ink is removed from the surface of one of the rollers by doctor blades.
It has been observed that during operation of an inking system wherein adjacent surfaces of rollers move in opposite directions, heat is generated at a rate which is related to the relative surface speed of the rollers, pressure between the rollers, and the lubrication between adjacent surfaces of the rollers.
Viscosity, surface tension, cohesion of ink molecules, and adhesion of ink molecules and molecules on surfaces of rollers are all related to temperature of the ink. Inking systems heretofore devised wherein ink was metered between adjacent surfaces of rollers moving in opposite directions have not controlled temperature of the ink or temperature of the roller surfaces. Since temperature was not controlled, several parameters which determined the thickness of a "metered" ink film were not controlled. Consequently, such systems were not capable of adjustment to continuously meter an ink film of a desired thickness.
Inking systems comprising rollers having adjacent surfaces moving in opposite directions have not included structure to control lubrication between the roller surfaces. Power required to drive a roller is a function of the torque and the speed of rotation of the roller.
Torque is force resisting rotation of the roller multiplied by the radius of the roller. The force resisting rotation is a function of the force urging the surfaces of the rollers together times the coefficient of friction. The coefficient of friction between lubricated surfaces depends both on the materials and conditions of the surfaces and on the lubricant.
If the speed of the roller is sufficiently low so as not to affect temperature of the roller surface, the coefficient of friction is substantially independant of surface speed. However, when the surface speed is increased and temperature of the lubricant and of the surface of the roller increase, the coefficient of friction decreases as the velocity increases.
It has been observed that in an attempt to meter a thin film of ink between rollers having adjacent surfaces moving in opposite directions, the coefficient of sliding friction between soft materials or between a hard material and a soft material is much greater than between hard materials. If a lubricating film between a hard surface and a soft surface, moving in opposite directions, is suddenly eliminated, frictional forces of sufficient magnitude to severely damage the soft surface may be encountered.
Heretofore no method of metering ink has been devised wherein ghosting is eliminated and a film of ink of regulated thickness is delivered to a printing plate for an extended period of time.